1. Field of the Invention
This invention relates to reducing capacitive coupling for transmission lines, particularly wires in submicron VLSI chips.
2. Description of Related Art
When multiple transmission lines (such as wires in semiconductor chips) are coupled in parallel between driving points and receiving points, capacitive coupling can occur between pairs of those transmission lines.
Capacitive coupling can cause crosstalk between pairs of transmission lines. Crosstalk between pairs of transmission lines can slow rise time of transmission lines which differ in logic value from nearby transmission lines; crosstalk between pairs of transmission lines can alternatively delay or speed rise time of transmission lines which have the same logic value as nearby transmission lines, causing delay of arrival of logic values in some of a plurality of transmission lines. In circuit design for a plurality of transmission lines, the designer must design the circuit to work even in the worst-case scenario; this can add unnecessarily to the design effort, or result in circuits which are inefficient or wasteful.
In deep submicron technologies such as 0.5 micron and below, wire resistance for transmission lines is larger, capacitance between the transmission line and the semiconductor substrate is larger, and the capacitance between pairs of transmission lines is larger. Capacitance between pairs of transmission lines can be over 80% of the total capacitance of the total capacitance for each transmission line. Capacitive coupling is severely exacerbated for mutliple parallel wires, particularly when those wires are longer than about 3 millimeters.
One known method for speeding up rise time of signals to be transmitted on transmission lines is to couple the transmission lines to stronger drivers. This can overcome the wire capacitance of the transmission line and decrease the time needed to change the voltage at the receiving point of the transmission line, and thus decrease the time needed to transmit a logic value from the driving point to the receiving point. However, the problems posed by capacitive coupling are not ameliorated by use of stronger drivers, due to wire resistance.
Moreover, the inventor has found that rise time can vary greatly in response to the timing of changes in different signals transmitted on pairs of transmission lines. Delay induced by capacitive coupling can be far greater when different signals on different transmission lines are switched at slightly different times, rather than simultaneously. Simulations show that a change in skew of only about 200 picoseconds can cause a change in the amount of delay by over 280 picoseconds.
A second known method for speeding up rise time is to provide repeaters at intermediate locations in the transmission lines. This can overcome some of the wire impedance and decrease the time needed to change the voltage at the receiving point of the transmission line, and thus decrease the time needed to transmit a logic value from the driving point to the receiving point. However, this method does not eliminate the problem because crosstalk still occurs on both sides of the repeater.
Accordingly, it would be advantageous to provide a technique for reducing capacitive coupling for transmission lines.